Years ago it was common if you filled a car with gas to the top, and then parked it in the sun for the afternoon, you would see the “expanded” gas leaked all over the pavement.
In today’s vehicles, filling a gas tank to the top means you run the risk of damaging your car’s evaporative fuel recovery system (charcoal canister). Some cars are more sensitive than others. Topping off today’s cars isn’t worth it.
The messages the OP refers to could be some sort of scam or spam.
As far as gas on a hot day goes; gas can expand and overflow a completely filled to the top gas can. When you fill up you are bringing up “cold” gas that is stored in a large tank perhaps 10 foot or more underground. The temp of the gas in the tank is in the 50’s. Once in the cars tank the gas can heat up, how much depends on the day and conditions. Hot air and even hotter pavement can heat the gas in the tank up to something close to 100 degrees in Arizona desert heat. A 50 degree increase in temperature will cause the gas in the tank to expand. In modern cars this expansion is captured in the tank and evaporative control system so fumes are not released into the air.
In old cars you could see the results of expansion in gas leaking out the filler pipe and onto the ground. You don’t see that anymore because the expanded gas is handled internally by the car, but damage to the evaporative control systems, primarily a damaged charcoal canister, can result. The result is a check engine light and an expensive repair to get it turned off so you can pass state emission testing
I suppose that if you are about to try to drive across the Gobi Desert, it might make sense to force extra gas into your tank. But, for most of us in the civilized world, the presence of gas stations every few miles and a functioning gas gauge make it…unnecessary…to top-off a gas tank.
As was said, in “the old days”, the worst that would happen is that you would waste some of that gas as it overflowed and scarred the asphalt below. Today, a stunt like this is likely to cost you several hundred $$ to replace the carbon canister that was contaminated by this bone-headed practice.
Bill Clinton might say something like .“it depends upon what you mean by a full tank”. IMO, it’s full when the first click of the nozzle shuts it off for you. It’s a pressurized system that is temperature dependent to some extent and does the adjustment for you. Just stop filling there, put the cap on, pay your bill and drive on. The tank is “full”. Everything changes volume when temperature changes. Don’t over think it. VDC is right.
@VDCdriver, I accidentally overfilled my tank a couple months ago. I was using up some stored gas before it got too old to use, and I actually did worse than what would happen during a normal top-off. Although I did have some difficulty filling the tank once after that, the charcoal canister eventually dried out, and everything is back to normal (thank goodness!).
It’s the people who frequently top-off their tanks who will likely spend hundreds of dollars for a repair. I seriously doubt doing it once is that likely to do permanent damage.
Don’t get me wrong, I am not defending the practice of topping-off. It’s stupid and sometimes costly, but I see people still doing it all the time. The pumps seem to be so sensitive these days that they save many idiots from themselves.
Bought an '84 Honda V45 Sabre in Nogal NM. Drove it home to Pittsburgh. Somewhere around Shamrock TX, I filled it up then did laundry for an hour or so.
When I came out, gas was seeping out of the overflow vent in the cap onto the outside of the tank.
OP, if you can schedule your fuel stops such that you drive 10 mi or so right after filling, you’ll burn off enough gas to handle any expansion.
This hot weather overfill thing is a myth, otherwise there’d be MILLIONS of damaged cars driving around Texas, Arizona, etc.
Just being hot isn’t a problem, anyway, it’s the temperature change the gas goes through. During the summer the gas starts out warmer, so relatively speaking it’s still about the same change.
“Topping off” is never a good idea, but as long as you stop when the pump handle stops your system is quite capable of handling the thermal expansion. It is designed to accomodate the increase in volume due to both that and due to agitation. Agitated fluids increase in volume.
By the way, you can blow up a barn easier with a few inches of gas in a barrel than with the barrel full. Light a match next to the bung hole of a full barrel and flames will spew out until the gas burns off. Ligh a match next to the bung hole of a barrel with a higly concentrated mix of gas and air and you’ll get a free ride to the moon. Please don;t test this at home.